1. Field of the Invention
The present invention generally relates to Global Navigation Satellite Systems (GNSS) such as GPS and GLONASS and, more particularly, to locating sources of noise and other signals potentially interfering therewith.
2. Description of the Prior Art
Aids to navigation have been considered essential to commerce, exploration, military endeavors and the like for many years. In recent years, satellite position locating systems such as Global Positioning System (GPS) and Global Satellite Navigation System (GLONASS) which provide world wide coverage have become available. Such systems require only satellite communication with self contained user sets which automatically calculate position in three dimensions using four or more satellites to a positional accuracy of only a few meters.
GPS has been used by the U.S. military for precision positioning and targeting for many years. Today its use is being extended to command/control, situation awareness, IFF (Identification Friend or Foe) and even individual position locators for personnel. In addition, as the uses of GPS have expanded, the cost of the user sets has fallen dramatically further increasing GPS availability. Today, the civilian sector has become the largest GPS market. In aircraft navigation, the precision available from GPS may even be exploited for instrument landings. Navigational uses are being extended to small commercial and pleasure boats and to automobiles in which a GPS receiver is interfaced with a computer to display vehicle position and direction on maps and interactively provide routing information.
The high orbits (altitude of approximately 20,000 km), the L-Band frequency, and the relatively low transmitting powers (hundreds of watts) used by GNSS satellites limit the strength of the radio signal which is received by user sets. Further, due to the need to simultaneously monitor transmissions from a plurality of satellites at large angular distances from each other, non-directional antennas are generally employed to receive GNSS transmissions. Accordingly, GNSS user sets provide very large signal processing gain for the satellite signal. However, the processing margins are low and interference can be a problem.
Due to the proliferation of uses and the increased reliance on GNSS, denial of GNSS service due to interference is becoming increasingly intolerable since it may unpredictably deny GNSS service to a user, particularly at a critical moment, as in an aircraft landing. Interference may take many forms and may be either intentional, as in jamming, or unintentional, such as improperly filtered FM radio and television broadcast signals. Industrial machinery may also cause interference if not properly shielded or properly operated. Intentional jamming is relatively easy and could be accomplished over substantial distances or areas by a relatively low power (e.g. 1-100 Watt) RF noise source. A jammer capable of transmitting a GNSS like signal would require only a fraction of such power to produce a similar result.
The combination of the low interference power levels needed to disrupt GNSS operation, the GNSS operating frequencies (.about.1.5 GHz) and the use of wide bandwidth noise-like signals can make location of the interference sources very difficult. However, location of the interference source must be accomplished to avoid effects of the interference or to provide the information needed to find and eliminate the interference source and restore GNSS service.
Due to the world wide nature of GNSS and the fact that potential interference sources could be found almost anywhere, an interference location solution that is part of the GNSS user set is desirable. Such a location solution would then be available where ever GNSS is being used and would not be constrained to particular platforms or installations. Further, a solution that can share existing parts of the user set can be added at minimal cost making it available to more users.